D.
If the vibrations have a low frequency, the sound is heard as a __low-pitched sound_____. If the vibrations have a high frequency, then the sound is heard as a __high-pitched sound_____.
Explanation:
High pitched sound is of higher frequency because they make the air molecules vibrate much faster than low-pitched sound. Due to their high frequency, their wavelength is shorter (Remember wavelength and frequency are inversely proportional). Vice versa is true to the low-pitched sound. They vibrate air molecules slower than high-pitched sound. Their wavelength is longer and frequency smaller.
Learn More:
For mor on pitch of sound check out;
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